The present invention relates to an auto-focus system, and more particularly, to a camera, camcorder, or similar apparatus incorporating an infinite-distance detecting circuit within an auto-focus system, such that the auto-focus system can precisely determine whether a "to-be-photographed" object is beyond a predetermined "infinite" focusing distance.
As typically employed in a camera, an auto-focus system accomplishes automatic focusing by detecting the distance between an object to the camera, and by adjusting the condition of the camera lens in accordance with the detected distance. In order to "detect" an object-to-camera distance, the auto-focus system typically emits a light beam, receives a portion of the light beam reflected from the object, and converts the reflected light beam portion into an corresponding electrical signal. The object-to-camera distance is detected by determining the relative magnitude of the resulting electrical signal.
The foregoing detection function is generally performed by a position status detector which is well known in the art. Upon detecting the object-to-camera distance, the position status detector generates a position detection signal S.sub.p. If the detected object-to-camera distance exceeds a predetermined "infinite" distance (for instance, eight meters), the auto-focus system recognizes the object-to-camera distance as being an "infinite" distance for focusing purposes.
FIG. 1 shows a conventional infinite-distance detecting circuit. The circuit shown in FIG. 1 includes an amplifier 5 having first and second inputs. Position detection signal S.sub.p is applied to the first input of amplifier 5 via an input 1. An input reference voltage V.sub.ref is applied to the second input of amplifier 5, such that the position detection signal S.sub.p is amplified in accordance with the ratio of selected resistors R1 and R2. The circuit further includes comparator 6 having a reference voltage potential largely established by the values of Zener diede ZD.sub.1, and resistors R4 and R5. Thus, the output of amplifier 5 is compared to a reference voltage potential at comparator 6 which generates a logical output, D.sub.o, of either "high" or "low" at output 2, depending on the relative levels of amplifier output and the reference voltage potential. Unfortunately, differences between V.sub.ref and the reference voltage potential create variations in the actual levels of the position detection signal interpreted as distance values exceeding the predetermined "infinite" distance. This imprecise detection of the "infinite" distance brought about by errant voltage tolerances degrades performance of the auto-focus system.